We previously have shown that muscarinic acetylchoine receptor activation causes a long-lasting loss of muscarinic binding sites in three different cell systems. In our present study, we plan to investigate the mechanism of neuroreceptor regulation in the neuroblastoma clone N1E-115. Our experiments focus on how receptor activation decreases the steady-state number of binding sites per cell as much as 90 percent. Using the specific binding of the muscarinic antagonist 3H-quinuclidinyl benzilate, we plan to measure changes in receptor concentrations in response to rigorously controlled independet variables. Our experiments are designed to answer the following fundamental questions: (1) What is the quantitative influence of receptor activation on the rates of receptor appearance into the cell membrane and of receptor disappearance from the cell membrane? (2) Is receptor binding site loss due to a reversible "masking" of receptors or is it due to loss of receptor molecules? (3) What is the role of metabolic processes and subcellular assemblies in regulation of neuroreceptors? In answering these three questions, our experiments additionally will provide data to test a new theoretical approach developed for analyzing the turnover of membrane proteins.